ELDONET – EUROPEAN LIGHT DOSIMETER NETWORK - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 97

them to disk in graphical form. The status bar at the bottom of the screen informs about
sunrise, sunset, system time, active window, path and filename of the current data file,
available disk space and the COM-port used by the ELDONET instrument. An icon
indicates the quality of the serial connection.
Menus allow printer setup and printing or copying the current graph. If WinDose
2000 detects an error such as communication problems or data exceeding the maximal
value the program issues visual and acoustic warnings to alert the user. Advanced
options allow indoor measurements not restricted by sunrise and sunset, dark current
measurements used during calibration of the instrument. In this mode the measured data
are recorded and stored in digital units. The depth readings can be set to zero in the
aquatic version to adjust for the air pressure at the measurement site.
All data are stored in ASCII files which facilitates further analysis of the data by
the analysis program developed for the project (WinData 2000) or by spreadsheet
programs such as EXCEL. The final record of the day contains a header with the
location information and shows the accumulated doses in the three channels for the day.
5. Calibration
Each radiation channel in every instrument is accurately calibrated against a
NIST traceable 1000 W quartz halogen calibration lamp with a constant spectral
composition operated with a highly stabilized power supply. All calibration factors are
stored in the individual .INI file. Furthermore, every instrument is tested under all solar
angles under blue sky in a location with free view of the horizon. The absolute
calibration was derived from an intercomparison of several spectroradiometers and the
ELDONET instrument in September 1997 in Garmisch-Partenkirchen (Southern
Germany). Instrument intercomparison is a vital tool for quality control and
documentation 43-44 . All instruments should be recalibrated at least every year.
Frequently ELDONET instruments are compared with double monochromator
spectrophotometers by running them side by side in parallel several days at different
geographic locations under different weather and irradiation conditions to warrant
absolute solar calibration.
In addition to cloud-related changes the solar spectral emission changes over the
day with less contribution of short wavelengths in the early morning and late evening
hours because of the different path lengths of the sun rays through the atmosphere. The
filter transmissions of the three channels are not perfectly square in the selected
wavelength bands, therefore the measured values need to be corrected in dependence of
the solar angle. This is done by direct comparison with a double monochromator
spectroradiometer for all possible solar angles during the daily cycle on clear days. This
check against the sun is carried out for all instruments, a procedure which is now
standard for most instrument manufacturers. The PAR and UV-A channels did not
deviate from the spectroradiometer values, but for the UV-B channel a correction
function was determined which is built into the program. The ELDONET data show
only minimal deviation from the spectroradiometer values. However, it is recommended
that the calibration procedure should be repeated at least every year.
Quality control and documentation is an indispensable prerequisite of
high-precision measurements of solar radiation. The resolution and detection limits of